The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height is on the order of Angstroms. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
Previously manufactured sensors have mainly consisted of what are called current in plane (CIP) GMR sensors. Such sensors are manufactures so that current flows from one side of the sensor to the other in a direction parallel with the planes of the materials making up the sensors. However, another type of GMR sensor that has received increased interest lately is a current perpendicular to plane (CPP) GMR sensor. A CPP sensor is designed so that current flows perpendicular to the planes of the materials making up the sensor (ie. from top to bottom). Electrical leads are formed at the top and bottom of the sensor and may be constructed of magnetic materials to also serve as magnetic shields. The ever increasing demand for improved data rate and data capacity has lead to increased interest in perpendicular recording systems. This has lead to an increased interest in CPP sensors which are particularly suited to use in longitudinal recording systems.
The ever increasing demand for improved data rate and data capacity has also lead to increased interest in Tunnel Junction Magnetoresistive sensors which promise to provide greatly improved magnetoresistance (dr/R) compared to currently manufactured GMR sensors. A tunnel valve includes a magnetic free layer and a magnetic pinned layer, similar to a GMR sensor. However, instead of having an electrically conductive spacer layer, a TMR sensor has a thin, non-magnetic, electrically insulating barrier layer. The TMR sensor operates based on the spin dependent tunneling of electrons through the barrier layer. This spin dependent tunneling varies with the relative orientation of the magnetic moments of the free and pinned layers.
Biasing of the free layer has traditionally been provided by hard magnetic layers formed at either side of the sensor. The hard magnetic layers, such as CoPtCr, couple with the outer edges of the free layer to provide just enough magnetic moment to bias the free layer without pinning it. While, the biasing at the center of the free layer is not equal to biasing at the edges, such biasing schemes have been adequate for prior art sensors. However the push for improved data capacity has required that sensors be made with ever smaller track widths in order to increase the number of tracks that can be recorded onto a disk. As the track width decreases the free layer becomes less magnetically stable. The prior art hard bias schemes become inadequate to maintain sufficient free layer stability.
The use of such traditional hard bias schemes is even more problematic when used in a CPP GMR or TMR sensor, since the electrically conductive hard bias layers must be insulated from the sides of the sensor stack to prevent current shunting. This requires depositing an insulation layer along the sides of the sensor and adjacent to at least one of the top and bottom leads. This separates the hard bias layer from the free layer further degrading the free layer biasing.
In order to overcome these drawbacks, engineers and scientists have sought to develop in-stack bias structures. Such bias structures are built into the sensor stack, such as above the free layer. An example of a sensor having such a prior art in stack bias structure is described with reference to FIG. 1. A sensor 100 has is described herein as a tunnel junction sensor (TMR), but could just as easily be a CIP or CPP GMR sensor. The sensor 100 includes a free layer 102 and a pinned layer 104 separated by a non-magnetic, electrically insulating barrier layer 106. A first AFM layer 108 pins the pinned layer 104.
The sensor 100 also includes first and second leads 110, 112. The leads 110, 112 can be constructed of a magnetic material to serve as magnetic shields as well as leads. A non-magnetic, electrically insulating gap layer 114, 116 extend from the sides of the sensor 100 filling the space between the leads 110, 112. A capping layer 117 such as Ta may be provided at the top of the sensor to protect the sensor layers, and seed layer 119 can be provided at the bottom to promote a desired crystalline growth in the sensor layers.
An in stack bias structure 118, formed over the free layer 102 includes a spacer layer 120, constructed of a non-magnetic, electrically conductive material such as Cu or Ta, formed over the free layer 102. A ferromagnetic bias layer 122 is formed over the spacer layer, and a second AFM layer 124 is formed over the bias layer. The AFM layer 124 pins the magnetic moment of the bias layer in a direction parallel with the ABS as indicated by arrow 126. Magnetostatic or exchange coupling between the free layer 102 and the bias layer 122 across the spacer layer 120 biases the magnetic moment of the free layer in a direction parallel with the ABS and in most cases antiparallel with the moment 126 of the bias layer as indicated by arrow 128.
As those skilled in the art will appreciate, in order for the sensor 100 to be operable, the pinned layer must have it's magnetic moment pinned in a direction perpendicular to the moment 128 of the free layer, such as indicated by arrowhead symbol 130. The pinned layer 104 could be an antiparallel pinned structure, but for purposes of simplicity is shown as a simple pinned layer. The pinned layer is pinned by exchange coupling with the first AFM layer 108.
Annealing the two AFM layers 108, 124 to set the pinned layer 104 and bias layer 126 in perpendicular orientations is problematic. The AFM layers are set by an annealing process that involves raising the temperature of the sensor 100 above the blocking temperature of the AFM material, applying a magnetic field in the desired direction, and then cooling the sensor 100 below the blocking temperature while the magnetic field is applied. However, if the magnetic field is applied in the desired direction to set the pinned layer, it will be in the wrong direction for the bias layer.
One way to overcome this is to use different AFM materials having different blocking temperatures and then doing two annealing steps, one for each AFM layer. However this requires that a less than optimal AFM material be used for at least one of the AFM layers 108, 124. For example if a desired AFM material, such as PtMn is used for the first AFM layer 104, a less than desirable AFM layer having a lower blocking temperature and poorer exchange coupling must be used for the second AFM layer 124. Also, the addition of a second annealing step increases manufacturing time, cost, and increases the potential for sensor degradation during the high temperature anneal.
Another problem exhibited by such prior art in stack bias structures is that they do not provide optimal biasing efficiency. If biasing efficiency is measured as the ratio of free layer moment to bias layer moment, the ratio for such a prior art in stack bias structure is only about 1 to 1.7. This means that relatively thick bias layer must be used to achieve a necessary bias moment. This increased bias layer thickness increases unstability of the exchange coupling between AFM layer 124 and biasing layer 122, increases parasitic resistance, and also increases gap height.
Therefore, there is a strong felt need for an instack bias structure that can provide strong, efficient free layer biasing without the need for a two step AFM annealing process. Such an in stack bias structure would preferably allow the AFM layers of the bias structure and pinned layer to be set in the same direction in a single annealing step and would also allow the use of AFM materials having similar blocking temperatures for both the bias structure and the pinned layer structure if desired.